Body implantable cardiac stimulators or pacemakers are known to the prior art. An early pacemaker is disclosed by Greatbatch in U.S. Pat. No. 3,057,356, entitled "Medical Cardiac Pacemaker", which issued in 1962. This device included a relaxation oscillator that generated electrical pulses at a fixed rate. The pulses were applied to the heart to cause the heart to contract each time a pulse occurred.
Since 1962, the pacemaker has been continuously evolving. This evolution is outlined in concurrently filed co-pending application Ser. No. 957,962, now U.S. Pat. No. 4,250,883, filed in the name of David L. Thompson for Digital Cardiac Pacemaker, which is co-owned with the present invention and which is hereby incorporated by reference. As noted in the incorporated specification, pacing technology has lagged behind conventional state of the art electronic technology in its utilization of digital electronic circuits. One reason for this has been the high energy required to operate digital electronic circuits. Energy requirements are a major concern in pacemaker design. However, with the continuing advances of electronic technology, digital electronic circuits are increasingly feasible within the context of commercial pacemaker units.
The accuracy and reliability of digital electronic circuits are factors that encourage their use within the pacemaker context. The facility with which they can be programmed and reprogrammed to alter one or more operating parameters further enhances their utility. For example, pacemakers have been disclosed which respond to magnetic and/or radio frequency signals to alter an operating parameter. Pulse rate and pulse width may be programmed in this manner. In addition, pacemakers have been constructed which are inhibited in the presence of certain signals. A more detailed outline of prior art programmable pacemakers is contained in the incorporated specification. It should be noted that, as indicated in the incorporated specification, no known prior art pacemaker is capable of having more than two parameters, features or tests programmed on command.
The implementation of digital electronic circuitry within the pacemaker context provides the opportunity to program or reprogram one or several operating parameters, on command, via externally generated signals. For example, pulse rate, pulse width and pulse amplitude can be externally established at one of any number of combinations. In addition, the refractory period may be established and altered. Further, digital circuitry can be programmed on a temporary or permanent basis, as desired. Of course, other operating parameters or characteristics can also be externally programmable.
Clearly then, a pacemaker utilizing digital electronic circuitry would have a more universal application by allowing the pacemaker to be programmed to fit the needs of a particular application as opposed to being manufactured for limited applications. In addition, such a unit can be instructed to give an external indication of its program status, particularly in instances where that status is not directly observable. However, even with the implementation of digital circuitry, certain analog circuitry is necessary to generate and/or transmit various control signals and to respond to the digital circuitry to effect its programming.